The disease caused by Haemophilus influenzae type b is a major cause of bacterial meningitis in children under the age of five years. Protective antibodies to the disease are induced by the capsular polysaccharide of the organism and a vaccine has been developed that utilises the purified polyribosyl ribitol phosphate (PRP) as the antigen. This vaccine gave 90% protection in adults and in children over 24 months of age, but was ineffective in children under 24 months. Like other polysaccharide vaccines, PRP does not induce the proliferation of T-helper cells, and re-immunisation fails to elicit either a booster response or an increase in memory cells. A new conjugate vaccine has been developed that uses the PRP linked to diphtheria toxoid (see European Patent No. 0,098,581), which elicits T-cell dependent, booster responses and the production of PRP-specific IgG antibodies. However, the recommendation of both the Immunisation Practices Advisory Committee and the American Academy of Pediatrics is that only children 18 months and older should be immunised using the vaccine, since the efficacy of the vaccine was inconsistent at the younger ages. To achieve universal protection in the 2 to 6 month age group and certain high risk groups, the incorporation of certain non-capsular antigens may be required.
Methods for inducing immunity against disease are constantly improving and there is presently a move to use smaller and better defined materials as antigens. This is being undertaken to minimise or eliminate potential side-effects due to certain native immunogens, while preserving their immunogenicity to confer protection against the disease.